Debugging is when you take your tamagotchi's battery apart and then you will have to do specific things to get your tamagotchi in debug mode. Once you are in debug mode, you can choose whatever tamagotchi you want. Beware of hyperspeed, though. Hyperspeed will never stop for the V3, but it will for the V1 and V2. I have not tested the debugging for the V4 or the V4.5, therefore I can not give you any information.
CAUTION: Debugging your tamagotchi may ruin your tamagotchi forever!